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Show 1881.] PROF. F. J. BELL ON THE GENUS ASTERIAS. 499 of greater value which neither specimens nor opportunities yet allow'. I now proceed to the consideration of the heads under which the numerous species of this genus are to be grouped. The genus Aster-acanthion of Muller and Troschel was divided by a single coup into a group with spines on the back, and a group with stalked knobs on the back ; those of the former group alone are now members of the emended genus. No other zoologist has, so far as I know, attempted any convenient grouping of the species, though Dr. Stimpson has rendered some assistance in this direction by pointing out the affinities of the species he describes. It is, at the same time, obvious enough that there is a very large number of species in the restricted and emended genus Asterias ; and the number is unwieldy even now, when our knowledge of the variations that may be found in it is very possibly altogether elementary. No good, however, is gained by hastily accepting or proposing ill-defined generic coups; the more necessary work just now is to sort out the species into different categories. These must be as natural as they can be ; but where nature fails us, or becomes too obscure for our vision, we must make use of what are not so satisfactorily natural characters. It is necessary to make some such preface as this, because I have to propose a primary subdivision of the genus to which, from purely theoretical considerations, it would be possible to raise some not un-i important objections ; for I propose, first of all, to separate the species into those in which there are developed more than five rays, and those in which, so far as we know, the number five is constantly retained. To these two groups I propose to apply the terms Heteractinida and Pentactinida2. It is, so far as is yet known, only among the former, or Heteractinida, that the presence of more than one madreporic plate has been noted, though in forms which belong to the Heteractinida, but in which five arms only are developed (and this is not only a possible, but an actual case) there may be, and at times are, two madeporites. The plurality of madreporic plates affords a good secondary point of difference; and I propose, therefore, to form subdivisions which may respectively be known as polyplacid and monoplacid. The third distinctive character lies in the number of spines which border the ambulacra-the adambulacral spines : as an ordinary rule, one finds one or two rows of these; and the forms which belong to them may be distinguished respectively as Monacanthida and Dipla-canthida. It is a difficult matter to say exactly what is the real significance of the difference in the number of the rows of adambulacral spines. In the first place, it is necessary that we should have for a number 1 This might almost be an echo of the words of Prof. Alex. Agassiz (see ' North-American Starfishes,' p. 122). 2 Whether this division be natural or artificial, it will, for the great majority be found, I think, to work pretty satisfactorily: the problem of heteractinism is still unsolved ; and it is a serious question whether the heteractinism of e. g. A. tenuispina is exactly comparable to the polyactinism of e. g. Heliaster. |